1. Field of the Invention
The present invention relates to the field of photolithography used in fabricating semiconductor devices and, more particularly to a method of eliminating the side lobe printing of attenuated phase shift masks.
2. Description of the Related Art
In the manufacture of semiconductor wafers, photolithography is used to pattern various layers on a wafer. A layer of resist is deposited on the wafer and exposed using an exposure tool and a template such as a mask or reticle. During the exposure process a form of radiant energy such as ultraviolet light is directed through the reticle to selectively expose the resist in a desired pattern. The resist is then developed to remove either the exposed portions for a positive resist or the unexposed portions for a negative resist, thereby forming a resist mask on the wafer. The resist mask can then be used to protect underlying areas of the wafer during subsequent fabrication processes, such as deposition, etching, or ion implantation processes.
An integral component of the photolithographic process is the reticle. The reticle includes the pattern corresponding to features (e.g., transistors or polygates) at a layer of the integrated circuit (IC) design. The reticle is typically a transparent glass plate coated with a patterned light blocking material such as, for example, Chromium. This type of reticle is typically referred to as a binary mask since light is completely blocked by the light blocking material and fully transmitted through the transparent glass portions.
There are problems with the binary mask. Light passing through the edge of a pattern within the mask (e.g., the boundary between a light blocking region and a transparent region) is oftentimes diffracted. This means that instead of producing a very sharp image of the edge on the resist layer, some lower intensity light diffracts beyond the intended edge boundary and into the regions expected to remain dark. Hence, the resultant feature shapes and sizes deviate somewhat from the intended IC design. Since integrated circuit manufacturers have continued to reduce the geometric size of the IC features, this diffraction produces wafers with incomplete or erroneous circuit patterns.
Attenuated phase shift masks (PSMs) have been used to overcome the diffraction effects and to improve the resolution and depth of images projected onto a target (i.e., the resist covered wafer). Attenuated PSMs utilize partially transmissive regions instead of the light blocking regions used in binary masks. The partially transmissive regions typically pass (i.e., do not block) about three to eight percent of the light they receive. Moreover, the partially transmissive regions are designed so that the light that they do pass is shifted by 180 degrees in comparison to the light passing through the transparent (e.g., transmissive) regions. Thus, some of the light spreading outside of the transparent region defined by the PSM pattern edge destructively interferes with light passing from the partially transmissive regions. This way, the detrimental effects caused by diffraction may be controlled.
FIG. 1a illustrates a portion of a conventional attenuated phase shift mask 10. The mask 10 includes a transparent portion 12 that permits transmission of radiant energy, such as ultra violet light, and phase shifting or attenuating portions 14 that only permit transmission of about three to eight percent of the light they receive. Also, the attenuating portions 14 phase shift any light they pass by 180 degrees. The attenuating portions 14 contain a single pattern or opening corresponding to a desired IC feature and is referred to herein as feature opening 16 (since a feature of the IC design will be produced from this opening in the attenuating portions 14).
FIG. 1b is a graph 20 illustrating the electric field amplitude, with respect to distance, present at a wafer being processed with the conventional attenuated phase shift mask 10 of FIG. 1a. As shown in the graph 20, the electric field profile actually contains three components: the first component 22, which is in phase with the light passing through the feature opening 16, and the second and third components 24, 26, which are 180 degrees out of phase with the light passing through the feature opening 16.
FIG. 1c is a graph 30 illustrating the light intensity amplitude, with respect to distance, present at a wafer being processed with the mask 10 of FIG 1a. As known in the art, intensity of the light passing through the attenuated phase shift mask 10 (FIG. 1a) is proportional to the electric field energy squared (i.e., I.varies.E.sup.2). As shown, the intensity profile includes a first component 32 corresponding to the feature opening 16 (FIG. 1a). This first component 32 is desired since it corresponds to a feature of the IC design. However, the intensity profile also includes two other components 34, 36 which are not desired. These components 34, 36 are the combination (sum) of the diffraction of the 180 degrees phases of the components 24, 26 (FIG. 1b) and the approximate six percent background of the attenuating portions 14 of the mask 10 (FIG. 1a). These components are known in the art as side lobes and may corrupt the desired feature or cause undesired features to be etched into the wafer (known in the art as side lobe effects).
FIGS. 1a-1c illustrate a simple mask 10 with only one feature opening 16. As known in the art, the side lobe effect becomes more pronounced as the spacing between the IC features decreases. That is, when features are designed close to each other, which is the current trend, the electric field and intensity components associated with the side lobes of each feature begin to overlap and add-up. This causes side lobes of greater amplitude and increases the side lobe effect. Sometimes, the amplitude of these "additive" side lobes is greater than the amplitude of the desired features, which further corrupts the fabrication process.
There is a need to eliminate side lobes from being printed from an attenuated phase shift mask. In theory, the light forming a side lobe can be eliminated by light that is 180 degrees out of phase with the side lobe light. One attempt at eliminating the side lobes, has been to manually place additional transparent openings in the attenuated phase shift mask at locations where it is believed that side lobes may be printed. The additional openings would be sized and formed in the reticle so that they will pass the proper amount of out of phase light to cancel the side lobe. This manual process, however, is extremely time consuming. In addition, since the feature size has dramatically decreased, and the number of features within the IC design has greatly increased, it is not feasible, and virtually impossible, to manually eliminate all of the side lobes particularly for a very large scale IC. Thus, wafers may still be ruined with a manually altered mask.
Other methods have used design rule algorithms to place the additional openings at locations where the rules detect that side lobes would print. Rules take the form of "if the distance between two features is X then a side lobe would print at Y." Developing the design rules, however, is very time consuming. In addition, an IC design may incorporate numerous unique environments (e.g., different placements of the IC features) and thus, the resulting attenuated phase shift mask may be very complex. Moreover, the likelihood that a side lobe will form and where it will form is dependent upon the configuration and proximity of adjacent IC features. It would be very time consuming and practically impossible to develop rules to locate all of the potential side lobes (and their locations) for the many possible feature environments within a complex IC design. That is, this method could not handle the full IC chip design.
Accordingly, there is a need and desire for a method of eliminating side lobe printing from an attenuated phase shift mask that is less time consuming then other attempts to eliminate side lobe printing. Moreover, there is a need and desire for a method of eliminating side lobe printing from an attenuated phase shift mask based on a full integrated circuit (IC) chip design.